There are two basic electrophotographic processes, the xerographic process and the electrofax process. In the former process, a photoreceptor, formed by applying a thin film of photoconductors such as selenium, cadmium sulfide and the like onto a metallic drum, is charged in the dark place and irradiated with a photo-image (exposing) to form an electrostatic latent image. From this latent image, a visible image is formed with a toner (developing) which is transferred to paper, etc., and fixed. In the latter process, a photoconductive layer (photosensitive layer) is applied onto paper, and a permanent visible image is formed on this layer through the steps of charging, exposing, developing and fixing.
As the photoconductor for electrophotographic photoreceptors now widely used, there are known inorganic compounds such as amorphous selenium, cadmium sulfide, zinc oxide, etc., and organic compounds such as polyvinylcarbazole (PVK), phthalocyanine, azo compounds, etc.
The organic photoconductor is superior in flexibility and processability, but it is not sufficient in electrophotographic sensitivity when applied alone for practical uses. This sensitivity is increased by applying chemical sensitization and optical sensitization in combination. The well-known chemical sensitizer includes for example, polycyclic or heterocyclic nitro compounds such as 2,4,7-trinitro-9-fluorenone (TNF), 2,4,5,7-tetranitro-9-fluorenone (TENF), etc., quinones such as anthraquinone, etc., and nitrile compounds such as tetracyanoethylene, etc. Also, the well-known optical sensitizer includes xanthene dyes and quinoline pigments.
Among the organic photoconductors, the phthalocyanine photoconductor gives excellent electrophotographic photoreceptors, but it has a defect in that, when exposed to severe environments, for example to high temperatures above 40.degree. C. high humidities above 80%, the phthalocyanine electrophotographic photoreceptor shows a great reduction in chargeability or a great deterioration in the electrostatic characteristics on repeated uses. As described above, the physical property of the photoreceptor changes greatly due to changes in the environment, so that the use of the photoreceptor sometimes causes a problem.